"Call to witness" is a phrase that can be replaced by a number of synonyms, each with its own unique connotations. One option is "summon to testify," which emphasizes the legal aspect of providing evidence under oath. Another synonym is "invoke as evidence," which suggests the act of using someone's testimony as support for a claim. "Beseech as a witness" implies a sense of urgency or desperation in requesting someone's testimony. "Request to testify" is a more straightforward and neutral alternative. Finally, "call upon to bear witness" has a formal, almost antiquated feel, but can be appropriate in certain contexts.